A flash memory which is a representative of a nonvolatile memory is incorporated into USB memories, memory cards, cellular phones, silicon audio, and others and is mainly used as a storage for storing motion picture data, music data, or digital data. In recent years, development has been made to provide higher-definition motion pictures or images and data capacities have been increasing. Under the circumstances, in view of alternatives of hard discs of note-type PCs, there has been an increasing demand for a larger capacity of the flash memory.
However, development of a larger capacity of the flash memory is approaching a limit, in terms of miniaturization and reliability. In place of the flash memory, new nonvolatile memories which can implement a larger capacity have been developed vigorously. Among the nonvolatile memories, there is a resistance variable memory (ReRAM: Resistive Random Access Memory). The resistance variable memory uses a resistance variable layer which changes its resistance values as a material of a memory element. The resistance variable memory change its resistance value from a higher-resistance state to a lower-resistance state or from the lower-resistance state to the higher-resistance state, by application of electric pulses (e.g., voltage pulses), thereby storing data. In this case, it is necessary to clearly distinguish two values corresponding to the higher-resistance state and the lower-resistance state, stably change the resistance state between the lower-resistance state and the higher-resistance state at a higher speed, and to retain these two values in a non-volatile manner. As an exemplary nonvolatile memory element, there has been proposed a nonvolatile memory element using a resistance variable layer including transition metal oxides which are different in oxygen content and are stacked together. Oxidization or reduction are caused to take place selectively in a region of an interface between a transition metal oxide layer with a higher oxygen content and an electrode in contact with this transition metal oxide layer and thereby a resistance changing phenomenon is stabilized (e.g., Patent Literature 1).
FIG. 16 is a cross-sectional view showing a resistance variable nonvolatile memory device 50 including a nonvolatile memory element 55 disclosed in Patent Literature 1. In the nonvolatile memory device 50 shown in FIG. 16, a first wire 61 is formed on a substrate 60, and a first interlayer insulating layer 62 is formed to cover the first wire 61. A first plug 64 penetrates the first interlayer insulating layer 62 and is connected to the first wire 61. Furthermore, a nonvolatile memory element 55 including a lower electrode 65, a resistance variable layer 66 and an upper electrode 67 is formed on the first interlayer insulating layer 62 to cover the first plug (conductive plug) 64. A second interlayer insulating layer 68 is formed to cover the nonvolatile memory element 55. A second plug 70 penetrates the second interlayer insulating layer 68. The second plug 70 connects an upper electrode 67 to a second wire 71. The resistance variable layer 66 has a stacked-layer structure including a first resistance variable layer 66x and a second resistance variable layer 66y which are stacked together. The first resistance variable layer 66x and the second resistance variable layer 66y of the resistance variable layer 66 comprise transition metal oxides of the same kind, respectively, and the transition metal oxide constituting the first resistance variable layer 66x has a higher oxygen content than the transition metal oxide constituting the second resistance variable layer 66y. 
In such a structure, when a voltage is applied to the nonvolatile memory element 55, a most portion of the voltage is applied to the first resistance variable layer 66x having a higher oxygen content and exhibiting a higher resistance value. In a region of the first resistance variable layer 66x, a plenty of oxygen used for reaction is present. Therefore, redox reaction takes place selectively in a region in the vicinity of an interface between the first resistance variable layer 66x and the upper electrode 67, and thus, stable resistance change is implemented.
Non-patent Literature 1 discloses a nonvolatile memory including a 1T1R (unit cell is composed of one transistor and one resistive element) using transition metal oxide as a resistance variable layer. Non-patent Literature 1 recites that a transition metal oxide layer is an insulator in a normal state, and breakdown (initial breakdown) occurs initially in the resistance variable layer to change the resistance value by application of electric pulses, thereby forming a conductive path which enables switching between the higher-resistance state and the lower-resistance state. The “initial breakdown” is defined as a process for changing the resistance variable layer after manufacture to transition reversibly between the higher-resistance state and the lower-resistance state, in response to an applied voltage or a polarity of the applied voltage. To be specific, a voltage (initial breakdown voltage) greater in magnitude than a write voltage is applied to the resistance variable layer after manufacture and having a very high resistance value, or a nonvolatile memory element including the resistance variable layer. The initial breakdown enables the resistance variable layer to transition reversibly between the higher-resistance state and the lower-resistance state, and to decrease its resistance value.